Liquid jet removal of plasma sprayed and sintered coatings

ABSTRACT

Gas turbine engine coatings must often be removed during engine maintenance and repair. The techniques utilized to accomplish this task, machining, chemical stripping, machining followed by chemical stripping, or grit blasting, frequently result in component damage or destruction. Liquid jet erosion can be utilized to remove seals, coatings, or portions thereof without damaging the engine hardware.

The Government has rights in this invention pursuant to a contractawarded by the Department of the Air Force.

This application is a continuation of Ser. No. .Iadd.08/189,860, filedFeb. 1, 1994, now abandoned; which is a reissue application of Ser. No.07/784,625, filed Dec. 5, 1991, now issued as U.S. Pat. No. 5,167,721;which is a continuation application of U.S. Ser. No..Iaddend.07/441,666, filed Nov. 27, 1989, now abandoned.

TECHNICAL FIELD

This invention relates to the removal of coating materials.Iadd.utilizing liquid jet erosion.Iaddend., and specifically to theremoval of abradable, wear resistant, and thermal barrier coatingmaterials which have been applied by either sintering powder or fibers,or by plasma spraying. ., utilizing liquid jet erosion.!..

BACKGROUND ART

Various types .Iadd.of .Iaddend.coatings and sintered materials are usedin numerous applications, such as in gas turbine engines to increaseefficiency and/or protect components from heat and wear. Types ofmaterials include thermal barrier coatings, abrasive coatings, abradableseals, and hard facing; hereafter referred to as coatings.

Since excessive blade/case clearances and disc/vane clearances withinturbine engines allow the escape of gases which decreases engineefficiency, an abradable seal can be applied to minimize the clearancesbetween the rotating and the stationary components. Thermal barriercoatings can be utilized to provide protection against hightemperatures, while abrasive coatings can be used to prevent detrimentalrub interactions and hard facing can be used to reduce wear.

Some coatings are applied to plasma or flame spraying; introducingparticles (usually powders) into a hot gas stream or flame(respectively) which causes the particles to splat onto the substratesurface where they adhere and build up as a coating. Application ofparticles (i.e. AB-1) or short wires (i.e. Feltmetal TM) onto asubstrate; by pre-sintering or partial sintering and then brazing, canbe used to produce abradable coatings comprised of bonded particles,wires, or powders and void spaces; while bond coats can be produced byplasma spraying or vapor deposition. Bond coats are usually used inplasma spray and vapor deposition applications; a bond coat being alayer of metallic composition applied to the substrate before thecoating is applied. U.S. Pat. Nos. 3,542,530, 3,676,085, 3,754,903,3,879,831, 3,928,026, and 4,704,332, (incorporated herein by reference)describe various coatings, while U.S. Pat. Nos. 3,413,136, 4,055,705,and 4,321,311 (incorporated herein by reference) describe applicationtechniques.

A common characteristic of these types of coatings is that the coatingstrength (cohesive strength) is relatively low; plasma sprayed orpartially sintered particles are not well bonded to each other and thereis usually porosity present. The strength of the coating is less thanthat of the substrate.

During engine maintenance, these coatings must frequently be removed; aprocess difficult to .Iadd.perform .Iaddend.reliably . .perform.!. andwhich frequently results in substrate damage. Various techniques havebeen employed for the removal of coatings: machining, chemicalstripping, machining followed by chemical stripping (see for exampleU.S. Pat. Nos. 4,339,282, and 4,425,185; incorporated herein byreference), and grit blasting. For example, machining followed bychemical stripping requires that the component be held stationary whilea machining tool removes the majority of the coating. A chemicalsolution, usually either a very strong acid or base, in then applied tothe coating surface to disintegrate the remaining coating material. Thistechnique requires extreme precision; without proper hardware alignmentduring machining.Iadd.,.Iaddend.damage to the substrate material occurs,while the chemical solution used tends to attack the substrate material.This process is also time consuming and labor intensive. Additionally,the chemical step, can produce hazardous waste. The individual processesof chemical stripping and machining also have the above describedproblems.

Another commonly used method, abrasive or grit blasting, also oftenresults in damaged or destroyed components. This process consists ofprojecting abrasive particles in a compressed air stream against thecoating. Since this technique requires immediate termination uponsubstrate exposure to prevent damage, it requires skilled operators.

Liquid jets above 10,000 psi, to the best of our knowledge, have notbeen utilized in the removal of coatings. Relatively low pressure liquidjets, 2,000 to 3,000 psi, have been applied in areas such as: cleaningapplications, nuclear contamination removal, concrete scarifying, andbarnacle and hull fouling removal, but not in an inorganic coatingremoval process.

Accordingly, an objective of this invention is to provide a convenient,cost effective, environmentally safe technique of removing coatings.

DISCLOSURE OF INVENTION

The present invention involves the removal of coatings utilizing aliquid jet erosion process. The liquid jet, while striking the coatingat an angle, traverses the region, removing the coating. Depending onthe liquid pressure, the liquid stream erodes the abradable seal/thermalbarrier with virtually no damage to the bond coat (if present), or canremove both the abradable seal/thermal barrier and bond coatsimultaneously without substrate damage.

The invention process can be used to remove plasma sprayed and sinteredcoatings whose cohesive strength is significantly less than that of thesubstrate.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a basic embodiment of this invention.

FIG. 1A is a cross-section of FIG. 1 which reveals the various layers ofthe coating.

FIG. 2 shows the results of utilizing a liquid jet removal process atvarying pressures.

BEST MODE FOR CARRYING OUT THE INVENTION

The removal of coatings using current techniques is a difficult, inexactprocess. It requires skilled technicians, a substantial amount of time,expensive equipment, and frequently, the component is destroyed.

The removal of the coating, bond coat, or both without damage to thesubstrate material can be achieved with a liquid jet erosion technique;making it a viable alternative to the prior art.

As previously mentioned, this invention uses a liquid jet erosionprocess to remove coatings. Critical parameters (see FIG. 1) include thenozzle distance from the coating, and the liquid pressure. Depending onequipment and pressure constraints, the nozzle can be placed up toapproximately 6 or even 12 inches from the coating surface, however,lesser distances are preferred, with 1/4 to 3/4 inch especiallypreferred.

The angle between the liquid jet and similarly the liquid contact andthe coating is a matter of preference. An angle of between 20° to 90°can be used, with an angle of between 30° and 90° preferred, and anangle of about 45° especially preferred (see FIG. 1). The angle, not acritical parameter, causes the liquid to remove the coating fragmentsfrom the region where the jet impacts the coating. The direction ofrotation effects the fragment location post-removal. It is preferred torotate the component such that it causes the liquid stream to movetoward the smallest angle formed between the liquid stream and thecomponent. Although this is merely a matter of preference, this rotation. .directions.!. .Iadd.direction.Iaddend.helps to remove the fragmentsfrom the interaction zone thereby ensuring that they to do not interferewith the process.

FIG. 1 is one embodiment of the invention. The liquid stream (5)contacts the coating (1) at the preferred angle, approximately 45°.Additionally, the component (10) rotates such that the liquid stream (5)moves toward the smallest angle between the liquid stream (5) and thecomponent (10) (see arrows (1A)).

The liquid stream can consist of any liquid having a viscosity between0.25 centipoise and 5.00 centipoise at 25° C. and 1 atm and which willnot damage the bond coat or substrate material, including water basedliquids. Higher viscosity liquids tend to present flow problems withrespect to spraying the liquid at high pressures, while lower viscosityliquids can be difficult to pressurize, possibly increasing equipmentcosts. Water, viscosity approximately 0.95 centipoise at 25° C. and 1atm, is preferred for reasons of cost and waste disposal. Additives,such as wetting agents, or various chemicals which will degrade thecoating without damaging the component, may also be useful.

A water jet pressure sufficient to remove the top coat and/or the topcoat and the bond coat is required. Since pressures greater than about60,000 psi will damage most gas turbine substrate materials, lowerpressures must be used. The optimum liquid pressure ranges from about20,000 to about 60,000 psi, with about 25,000 to about 40,000 psipreferred. The factors which determine the exact pressure requiredinclude the type of top coat and if the coating is to be removed down tothe bond coat or to the substrate. (see FIG. 1A; top coat (1) and bondcoat (2)). Exact pressure limits are also related to nozzle geometry andspacing, and to the specific substrate involved. In practice, theskilled artisan can readily determine the pressure which causessubstrate damage and/or the pressure which causes bond coat removal andreduce this pressure to arrive at a suitable process pressure.

FIG. 2 shows the effects of varying pressures when using this invention.As the pressures decreased, from run (A) to (D), the amount of sealremoved also decreases, to the point where the abradable seal/thermalbarrier is removed with virtually no damage to the bond coat, (D).

This invention will be made clearer with reference to the followingillustrative examples.

EXAMPLE 1

The following procedure is used to remove a plasma sprayed hard facecoating, top coat and bond coat, (consisting of 20 v/o of an 80 nickel,20 chromium alloy, balance chromium carbide) from a substrate material.

1. The coated substrate material is arranged such that relative motioncan be produced between it and the water jet nozzle.

2. The water jet nozzle is placed so that the exit end of the nozzle isabout 1/4 inch from the coating and the water stream contacts thecoating at an angle of 45° (refer to FIG. 1).

3. The water pressure is 40,000 psi.

4. Relative motion is created between the water stream and the coatingsuch that as the coating is removed the component advances to the nextregion to be removed.

5. The removal time is dependant upon the surface area of the coating.The time will range from 5 minutes to 10 minutes for typical gas turbineengine components.

EXAMPLE 2

A sintered abradable coating (consisting of approximately 65 v/o nickel,35 v/o chrome, balance aluminum) can be removed by following thespecifications set forth in Example 1, while substituting a pressure of35,000 psi for the 40,000 psi in step 4.

This process can be used for any coating which has strength less thanthat of the substrate, by adjusting the pressure such that it removesthe coating without bond coat damage, or the bond coat without substratedamage, allowing reuse of the bond coat and substrate or the substraterespectively.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the claimed invention.

We claim:
 1. A method for removing a top coat from a bond . .coating.!..Iadd.coat .Iaddend.adhered to a substrate, utilizing a liquid jet, saidliquid jet having means for directing the liquid jet, means for creatingsufficient pressure to remove the . .coating.!. .Iadd.top coat.Iaddend.,means to provide the relative motion between the . .coating.!. .Iadd.topcoat .Iaddend.and the liquid jet, and means for supplying the liquid,which comprises:a. creating sufficient pressure to remove the ..coating.!. .Iadd.top coat.Iaddend.; b. providing relative motionbetween the . .coating.!. .Iadd.top coat .Iaddend.and the liquid jet; c.supplying the liquid; d. causing the liquid to strike the top coat,wherein the liquid striking the top coat causes top coat erosion untilthe bond coat is exposed;whereby the bond coat and the substrate sufferessentially no damage and can be reused.
 2. A method as in claim 1wherein the top coat is selected from the group of plasma sprayed, flamesprayed, and sintered coatings.
 3. A method as in claim 1 wherein thetop coat is an abradable .Iadd.coating.Iaddend..
 4. A method as in claim1 wherein the top coat is a thermal barrier .Iadd.coating.Iaddend..
 5. Amethod as in claim 1 wherein the top coat is an abrasive.Iadd.coating.Iaddend..
 6. A method as in claim 1 wherein the ..coating.!. .Iadd.top coat .Iaddend.is a hard facing.Iadd.coating.Iaddend..
 7. A method as in claim 1 wherein the liquidpressure is from about 20,000 psi to about 60,000 psi.
 8. A method as inclaim 1 using a nozzle as the means for directing the liquid flow.
 9. Amethod as in claim 1 wherein the liquid is selected from the group ofliquids consisting of all liquid which does not degrade the bond coat,and has a viscosity between about 0.25 centipoise and about 5.00centipoise at 25° C. and 1 atm.
 10. A method as in claim 1 wherein theliquid is selected from the group consisting of water based liquids. 11.A method as in claim 1 wherein the liquid is essentially water.
 12. Amethod as in claim 1 wherein the angle between the liquid stream and thetop coat is between 20° and 70°; whereby the angle causes the liquidstream to clean away the . .coating.!. .Iadd.top coat.Iaddend.fragments.
 13. A method . .as in claim 1 further comprising thestep of removing the bond coating, wherein.!. .Iadd.for removing acoating, said coating comprised of at least a top coat and a bond coatadhered to a substrate, which comprises: directing a pressurized liquidjet at a pressure above about 20,000 psi at the coating such that saidliquid jet strikes said coating, thereby removing said coating from thesubstrate, whereby .Iaddend.the substrate . .material suffersessentially no damage.!. .Iadd.may be reused.Iaddend.. .Iadd.
 14. Amethod as in claim 13, wherein said top coat and said bond coat areremoved simultaneously..Iaddend..Iadd.15. A method as in claim 13,wherein said liquid jet pressure is between about 20,000 psi and about60,000 psi..Iaddend..Iadd.16. A method for removing a protective coatingapplied to a substrate which comprises: directing a liquid jet at apressure above approximately 20,000 psi at the protective coating suchthat the liquid jet strikes the protective coating thereby removing theprotective coating from the substrate whereby the substrate may bereused..Iaddend..Iadd.17. A method as in claim 16, wherein theprotective coating is a thermal barrier coating..Iaddend..Iadd.18. Amethod as in claim 16, wherein the protective coating is an abrasivecoating..Iaddend..Iadd.19. A method as in claim 16, wherein theprotective coating is an abradable seal..Iaddend..Iadd.20. A method asin claim 16, wherein the protective coating is a hardfacing..Iaddend..Iadd.21. A method as in claim 16, wherein the liquidjet pressure is between about 20,000 psi and about 60,000psi..Iaddend..Iadd.22. A method as in claim 16, wherein the protectivecoating was applied by a pre-sintering and brazing or a partialsintering and brazing process..Iaddend..Iadd.23. A method as in claim16, wherein the protective coating was applied by a partial sinteringand brazing process..Iaddend.